Add fuzzing harness for various `CScript` related functions.

**Testing this PR**

Run:

```
$ CC=clang CXX=clang++ ./configure --enable-fuzz --with-sanitizers=address,fuzzer,undefined
$ make
$ src/test/fuzz/script
…
# And to to quickly verify that the relevant code regions are triggered, that the
# fuzzing throughput seems reasonable, etc.
$ contrib/devtools/test_fuzzing_harnesses.sh '^script$'
```

`test_fuzzing_harnesses.sh` can be found in PR #17000.

cmake -GNinja .. -DENABLE_SANITIZERS="address;fuzzer" -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++
ninja bitcoin-fuzzers
./src/test/fuzz/script